ENGINE MOUNTINGS.
The mounting of the engine and the general arrangement constitute
one of the most important and interesting sections of aeroplane
construction, and perhaps a brief outline of the various engines in
use will suitably preface a consideration of the mountings of the
different types. Although there are signs that certain revolutionary
engines may eventually come into use, the types in use on modern
aircraft are the stationary air and water-cooled, the radial air and
water-cooled, and the air-cooled rotary. The greater variety occurs
with the stationary type of engine, which may be sub-divided into those
in which the cylinders are arranged vertically in line, and those where
the cylinders viewed from the front form a =V=. Engines typical of
the former class are the Beardmore, Green, Mercedes, and Benz, all of
which are water-cooled; and of the latter class, Rolls-Royce, Sunbeam,
Hispano-Suiza water-cooled, and the Renault and R.A.F. air-cooled. The
types of radial engines which have been extensively used are confined
to two, these being the air-cooled Anzani and the water-cooled Salmson.
There is another radial engine of comparatively recent production; but
mention of this while present conditions obtain is not permissible.
Rotary engines of note are the Gnome, Le Rhone, and Clerget, all of
which are necessarily air-cooled.
Essential Requirements of an Engine Mounting.
The essential features of any mounting are absolute rigidity,
accessibility to permit ease of erection and dismounting; and it
should also be of a moderately low weight. Moreover, the general
arrangement must offer a minimum of head resistance, although in this
direction the type of engine used is a determining factor. Rigidity is
a paramount consideration, for the slightest tendency to slackness or
“play,” under the effect of engine vibration, speedily develops, until
either serious stresses are induced in the fore part of the fuselage or
the engine loses its correct alignment, with a consequent detrimental
effect on the flying qualities of the aeroplane. This, of course,
should be provided against in the general design; but it is also a
contingency which should be kept in mind during the actual construction
of the various components of the complete mounting.
A detail which does not always receive sufficient attention is the
provision of adequate bracing against the thrust of the engine. Where
the construction is such that the engine-bearers form an integral part
of the fuselage structure, there is generally little fault to find, but
with some sheet steel mountings, particularly those employed for the
rotary type of engine, the only bracing in a fore-and-aft direction
is that provided by the flanged edges of the plate, which are usually
much too narrow to be of real use. Further, the construction of both
the engine mounting and the fore part of the fuselage should be of the
necessary strength to ensure that the bearers supporting the engine
are always correctly in alignment and dead level. With some methods
of construction the weight of the engine and various landing shocks,
result after a time in the lowering of the bearers at the front, which
means that the angle of thrust is not in its correct position relative
to the centre of gravity and the incidence of the wings, this being
extremely detrimental to the flying properties of the machine.
Materials.
Wood, on account of its property of absorbing vibration, is
particularly well suited for the construction of the engine mounting,
and one finds examples of its use in a variety of ways. Perhaps
the most common form is that in which a bearer of ash or spruce,
channelled out between the fastening-down bolt holes for lightness,
is attached to steel brackets which in turn are bolted to the various
fuselage members. A development of this method consists of mounting
the bearers on either multi-ply formers of wood, or built-up wood
brackets stiffened with a three-ply covering on each side, and both
of these arrangements are being extensively used. Sheet steel is used
for the mountings of various machines, but it does not possess the
characteristic of absorbing vibration. In some instances one finds that
the engine has been specifically designed to be supported on bearers
of the tubular variety, in either steel or duralumin; but here again
rigidity is difficult of attainment. Although the use of welding,
that is to any extent, is not advisable in the construction of the
engine mounting, one finds this process very extensively used for the
mountings of some modern machines. In one particular instance, the
tubular bearers are supported from the steel tube fuselage by various
tubes, the whole structure being welded, and although every joint
successfully survived a smash which resulted in a considerable bending
and distortion of the fuselage, its use does not engender a sense of
security or reliability.
Rotary Engine Mountings.
[Illustration: FIG. 117.--Rotary engine mounting, in which engine is
supported between two plates.]
The mountings associated with the rotary type of engine fall under two
categories: those where the motor is supported between two or more
plates, and those in which the motor itself is overhung. The method
of mounting adopted for the first case is generally the type shown
by Fig. 117. The plates are pressed or bent up from sheet steel, and
all edges flanged to prevent buckling. The front plate embodies a ball
race, through which the propeller shaft runs, while to the rear bearer
is bolted the back plate of the engine. This arrangement with minor
variations has been extensively used for the different makes of small
scouting biplanes engined with the 80 h.p. and 100 h.p. Gnome motors.
Where the weight of the rotary engine used is excessive, as in the case
of the 160 h.p. Gnome with 20 cylinders, which is now out of date, a
mounting incorporating three bearers is used. The arrangement would be
similar to that indicated by Fig. 117, with the addition of an extra
bearer for the support of the crank-shaft extension.
Overhung Mounting.
[Illustration: FIG. 118.--Overhung rotary engine mounting.]
The overhung type of engine mounting which is used for both propeller
and tractor aeroplanes, is shown by Fig. 118. In this case the back
plate of the motor is bolted to the capping plate, while an extension
of the hollow crank shaft is supported by a smaller rear plate.
This system has been very widely used, chiefly by reason of its
extreme lightness, and the great facility afforded for the operation
of dismounting the engine; indeed, it would be difficult to find
an arrangement in which the demands of accessibility are so well
satisfied. Another form of overhung mounting, which has been used for
a radial Anzani motor, is shown by Fig. 119. In this case the four
longerons of the fuselage are capped by a single flanged steel plate,
to which the engine is attached by long bolts through the crank case.
Additional support is provided by light steel tube stays, which are
taken from various points on the front of the crank case to the centre
section of the upper plane, or other parts of the machine.
[Illustration: FIG. 119.--Anzani type engine mounting.]
[Illustration:
_FRONT ELEVATION_ _SIDE ELEVATION_
FIG. 120.--An overhung mounting built up of steel tubes.]
A distinctly original type of overhung mounting is shown by Fig. 120
in front and side elevation, this being used on a machine incorporating
an all-steel fuselage. The ring to which the back plate of the engine
is bolted, is supported from the four corners of the fuselage by steel
tubing, while the bearing for the crank-shaft extension is formed by
a pyramid of tubes welded to a pressing of sheet steel, to which in
turn is bolted a ball-race housing. At each corner the three converging
tubes are welded together, and bolted to small angle plates, which are
also welded to the framework of the fuselage. It will be seen that the
strength of this mounting is entirely dependent upon the welding; but
such reliance, in view of the generally uncertain nature of this latter
process, is not to be recommended.
A Stationary Engine Mounting.
[Illustration: FIG. 121.--Stationary engine mounting.]
A mounting used for a 70 h.p. air-cooled Renault, which is designed to
be supported by short lengths of steel tube projecting from the crank
case, is shown by Fig. 121, this particular arrangement being used on a
propeller biplane. The four ash longerons of the nacelle are built up
in the form of a box girder, the struts immediately under the engine
bearers being reinforced with steel plates. The steel tubes from the
crank case embedded in a steel bearing, composed of two semi-circular
clips, which are let into the upper longerons, and are prevented from
moving sideways by steel collars sweated to the tubes and abutting
against the fixing clips. In this case, by the removal of the four
fixing clips and the necessary pipe connections, the engine can be
lifted bodily out.
Multi-Engine Mountings.
Several versions of the type of machine employing two or more engines,
and which, by the way, is regarded as being the type most suitable
for the commercial purposes of the future, are existent. The usual
arrangement with the twin-engined machine is to support the engines
between the planes on either side of the body, the bearers being
mounted on a structure of struts, which also serve as interplane
supports.
With regard to the flying-boat type of machine, a favourite practice
is to mount the motor on the four struts supporting the centre section
of the upper plane, which is braced by struts and wires from different
points on the hull or body.
Although the modern aircraft engine is of greatly increased power,
compared with the engine of the period 1912–1914, one does not find any
great difference in the structural features of the mounting employed,
and in view of the very diverse arrangements for mounting the same type
of engine which now exist, there is need for greater uniformity. With
regard to the materials employed, there is a very pronounced trend
towards the greater use of wood, which circumstance is certainly at
variance with the oft-portended approaching era of steel. As indicated
in previous chapters, wood possesses remarkable powers of resistance to
sudden shock, which, combined with its quality of absorbing vibration,
renders it peculiarly suitable for the structure of the aeroplane,
and despite its numerous defects, will undoubtedly continue in use
until either the available supplies of suitable timber are exhausted,
or until the production of a remarkably light alloy possessing high
strength values.